Existing Bridge Failures: Implications for Multi-Hazard Risk Management

Bridges represent the link that guarantees continuity of roads in normal and emergency situations. Roads are the arteries through which the economy pulses, and in normal operations a bridge grants the continuity of roads and of the related economy. Bridge failure causes the immediate economy loss due to the stop of the activities, but unavailability of bridges during emergencies hampers the mobility of people that need to be evacuate and of teams that try to reach the disaster area. The cost of bridges unavailability grows exponentially during emergency situations, necessitating special attention. Hydraulic phenomena account of over 50% of bridge failures (e.g. Montalvo et al., 2020), and scouring around piers and abutments always causes serious damages if proper deepening of foundation is not provided in the design. Improper pier-foundation geometry and alignment with respect to flow direction further exacerbate these risks. Lacks in scientific and technical knowledges have led in the past to inadequate piers and foundations, and this fact joined with the increased frequency of hazardous events due to the climate change amplifies the risk of cascading events. Moreover elongated piers lead to an increase of scour depth even in case of moderate events, being always present a skew angle with respect to the local flow direction. The physical processes to which scour is attributed has been extensively studied to understand the influence of pier-foundation geometry and alignment on the multi-risk of failure. This is done by physical modelling of the sediment-flow-structures interaction, that leads the possibility to investigate pier-foundation geometries, skew angles and flow depths. The experiments have been developed in a rectangular flume 1 m wide and 15 m long, using quite uniform sands (median grain size d50=0.4mm) to reproduce the riverbed. Different pier-foundation geometries and water depths are considered in the experiments in steady state clear water conditions, respecting the Froude and Shields similitudes and evaluating the scour evolution in time. The variability of the depth and duration of the flow are expressed in terms of probability of the forcing conditions that lead to a scouring phenomenon for each investigated pier-foundation geometry, which can therefore be characterized by a specific hazard.